The present invention is with respect to an overpressure or gas let-off valve, more specially for flexible packages, having a valve body designed to be joined to the wall of such a package fluid-tightly and an elastic diaphragm for shutting off valve openings in a valve seat in the valve body or housing, the valve openings being at the end of holes running through from one side of the valve body to an opposite side thereof.
A valve on these general lines is to be seen in German Offenlegungsschrift specification No. 2,549,855 in the name of Wipf AG Verpackungen.
Such valves are used more specially for flexible packages which, after being filled with foodstuffs, are heat-sealed for safeguarding the foodstuffs against the effects of air, oxygen and humidity when they are being transported and stored. If the foodstuffs are such that the oxygen present in the package is likely to be the cause of a drop in quality, such packages may be exhausted for freeing them of oxygen and then, if desired, filled with an inert gas, or they may be swilled out with a counter-current of inert gas for clearing them of such oxygen from the air.
In the case of roasted coffee trouble conditions are likely, more specially, in the case of ground coffee, inasfar as such coffee has the property of giving up large amounts of gas between two and thirty days after roasting, such gas being mainly carbon dioxide. If roasted coffee is stored in gas-tight packages of the sort noted, there will be a danger of the package bursting because of the building up of a gas pressure as the gas is let off by the coffee. For this reason, such packages have been designed with overpressure valves so that there is no chance of a pressure building up, see for example German Offenlegungsschrift specification 1,903,048, which is with respect to the use of an overpressure valve with a flexible package for this very purpose.
Such valves have to be such that they may be used in large numbers and, for this reason, have to be able to be produced at a low price. A further condition to be kept to by such a valve is stopping oxygen from the air making its way into the package; putting it differently, the valve has to be a check valve which is completely gas-tight in the one direction, that is to say from the outside to the inside. At the same time, the valve design has to be such that pressure inside the package is safely let off as from the very lowest valve opening pressure, that is to say gage pressure, inside the package in an outward direction. After the valve has been opened by the overpressure and the gas let off, the valve has to be able to go back automatically into its shut position, frequent opening and shutting operations, or at least a small number of such operations having to take place in a completely safe way so that there is still no chance of oxygen from the atmosphere making its way into the valved package. Furthermore, the valve has to be so designed that there is little chance of it being damaged by blows or the like, for example on transport and on loading and unloading of such packages.
The prior art valve of the sort noted does in fact keep to some of these conditions inasfar as it may be cheaply made of thermoplastic material. In this known valve, the valve seat within the valve body has a middle frusto-conical part, over which the diaphragm is stretched, the diaphragm itself having a gas let-off opening shut against the seat. Furthermore, at its outer edge, the diaphragm is welded onto the valve body. Along the edge of the frusto-conical middle part, used as the valve seat, there is a number of holes through the valve body running into the inside of the package when the valve is fixed in the package valve. The frusto-conical middle part furthermore has a number of concentric, radially running channels within which there is a sealant, more specially a silicone oil, the radially running channels starting at the holes at the edge of the valve seat and ending at the narrow end of the frusto-cone in a ring-channel placed round it. When the diaphragm is stretched on the valve seat, this ring-channel is placed round the hole in the diaphragm. In this case, in one form of the invention, the material of the wall of the flexible package may be used as the diaphragm, if it has a hole for this purpose.
Although, as we have seen, this known valve keeps to a large number of the conditions of good overpressure valve design, it does, however, at the same time, have certain shortcomings. One important shortcoming to be seen on general of the valve is that there is likely to be some trouble on making a good weld between the diaphragm and the valve body and the wall material of the package. Furthermore, this further operation--welding together--is likely to be an important price factor at the scale of production coming into question. Because the diaphragm forming material is fixed at its edge, when the known valve is worked by the overpressure, the elasticity of the material has to be overcome so that generally high opening pressures are necessary. Even if the diaphram is placed loosely over the frusto-conical part, friction in a radial direction has to be overcome for forming outlet paths running to the hole through the diaphragm. This is a further effect increasing the pressure needed for opening the valve. However, one of the conditions to be kept to by such a valved package to make certain of trouble-free operation is that the opening pressure is to be as low as possible. Because, furthermore, the holes running through into the inside of the package in the valve body are, generally speaking, grouped together at one point, it may further be the case that, when the package is used, because of dust-like material such as coffee powder, these let-off holes will become stopped up and there will be trouble on operation of the valve.